The present invention relates to prosthesis materials for artificial bones or teeth and particularly to a plastic implant and a method for fabricating such a plastic implant to have a controlled surface porosity, with pore size depending upon the nature of tissue ingrowth desired.
In the related fields of dental implantology and orthopedic and surgical repair and replacement, numerous implant materials have been used with varying degrees of success. Metallic implants and prostheses, for example, have been found to have drawbacks due to chemical and electrolytic reactions, body rejection, toxic response, metal fatigue and failure, interference with healing, and inability to bond with bone. More recently, ceramics and polymers have been used with improved results insofar as bodily reaction is concerned, but problems still remain in developing permanent fibrous connective tissue and/or bony attachment (fixation) at the implant site.
Providing an implant with a porous surface at its interface with bone or other tissue has been recognized as promoting a firm union between the implant and the adjacent membrane in which it is embedded. For example, U.S. Pat. No. 3,628,248 issued Dec. 21, 1971 to E. A. Kroder et al. describes a procedure for preparing a tooth root replica implant from a mixture of polymethyl methacrylate beads and potassium chloride granules in a weight ratio of approximately 4:1. The powder mixture is combined with liquid monomer and a small amount of benzoyl peroxide (to initiate polymerization) in a mold for self-curing. After removal from the mold, the root implant is placed in boiling water for about 1 minute to extract a portion of the potassium chloride, thereby producing porosity.
As pointed out, however, in U.S. Pat. No. 3,713,860, issued Jan. 30, 1973 to A. Auskern, residues of additives to methyl methacrylate monomer, such as inhibitor, catalyst, or promoter, may cause tissue reactions at the implant site, and the benzoyl peroxide initiator of Kroder is known to be toxic to human tissues. The Auskern patent discloses a porous ceramic aluminum oxide bone substitute that is impregnated with pure methyl methacrylate (MMA) monomer. The monomer is then polymerized by gamma radiation from a suitable source, such as cobalt-60. If not previously shaped to size, the plastic-reinforced ceramic body is then machined, and the areas where bone or tissue ingrowth is desired are exposed to a suitable solvent (e.g. acetone) in an ultrasonic bath for 15 to 30 minutes to dissolve the polymer plastic to a preferred depth in the range of 100 to 400 microns.
According to Auskern, connective and bone tissue will grow into the pores and be firmly bonded to the ceramic if the porosity of the ceramic is in the range of 30-50 percent by volume, with the greater number of pores being in the range of 75 to 150 microns in diameter. On the other hand, a paper published by the present applicant in The New York Journal of Dentistry, Vol. 42, No. 10, pp. 331-341, December, 1972, suggested that the optimum porosity size for attachment between the periosteum and a polymer plastic tooth implant might fall in the range of 150-450 microns.
Another investigator has suggested a prosthesis material useful for artificial bones or teeth in the form of a heat-consolidated body composed of an integrated mixture of discrete microcrystals of calcium phosphate and a refractory compound such as aluminum oxide (see U.S. Pat. No. 3,787,900 issued Jan. 29, 1974 to T. D. McGee). In using this material for dental implants, McGee suggests providing porosity for the base of the teeth, where they fit in the alveolar sockets, by adding a volatile material such as naphthalene crystals to the dry ingredients at the root end of the implant mold before pressing and firing. In one example, forty percent naphthalene crystals were incorporated into one half of a cylindrical specimen to produce 200-500 micron pores in one end after firing. The specimen was implanted into the jawbone of a dog. A very strong bond, believed to be mineralized bone, developed between the implant and the jaw after 10 weeks.
Considerable work on dental implant material comprising an acrylic polymer mixed with organic bone has been done by M. Hodosh. See, for example, U.S. Pat. Nos. 3,609,867 and 3,789,029. In the latter patent, Hodosh describes a process for obtaining increased porosity near the outer surface of the polymer implant, where such porosity is most beneficial, by adding a measured amount of N-tributyl phosphate (a blowing agent) to a mixture of polymethacrylate, grated bone, and a foaming agent. The total mixture is polymerized in a mold at 300.degree. F. for 30 minutes. After removal from the mold, the processed implant has an outer skin that must be removed by suitable procedures, such as sand-blasting, to expose the porosity caused by the N-tributyl phosphate. According to Hodosh, when the prosthesis is implanted, tissues penetrate the pores to effect a secure fibrous interlock between the implant and the surrounding periodontal membrane. With such a blowing agent no predictable or standard pore size can be achieved, however.
Despite the progress in ceramic and polymer plastic implants and prostheses outlined above, there still exists a need for a plastic implant having standardized pore size (e.g., a desired hole, any dimension, uniformly distributed, every time) and with no toxicity. In addition, there is a need for a variety of porous plastic implant designs adapted for use in widely different jawbone configurations where no teeth exist (for example, narrow ridge implants, implants near maxillary sinuses, and implants in regions of inadequate alveolar height).
In particular, there is a need for a method of fabricating such an implant that can be performed quickly and easily by a dental or orthopedic surgeon or technician during an operating situation when the allowable time between the removal of a tooth or other surgical procedure and the fitting of the completed implant is measured in minutes.